Although a large number of studies have looked at different aspects of immune function in transplant recipients, relatively little information is available concerning the effect of blood transfusion on different lymphocyte subpopulations. This study will examine the effect on the immune system of transfusions matched and mismatched for class I and class II HLA antigens in patients undergoing cardiac surgical procedures. At this time, we are currently still in the pilot study phase of the protocol. When we have gathered 10 patients who have received blood transfusions, a data analysis will be performed in order to predict sampling schedules for the patients in phase II. Currently 7 patients who have been transfused have been studied. We anticipate the pilot phase to be completed in December, 1988.